This invention relates to Schottky rectifiers, and more particularly to measures for increasing the breakdown voltage of such rectifiers. The invention also relates to methods of manufacturing such rectifiers.
Schottky rectifiers are known comprising a semiconductor body having a body portion of one conductivity type between first and second main electrodes, of which the first main electrode forms a Schottky barrier with the body portion at a plurality of rectifier areas of a first surface of the body portion. Various embodiments of such rectifiers are disclosed in U.S. Pat. No. 4,646,115 (our reference PH33047), the whole contents of which are hereby incorporated herein as reference material. In one type of embodiment, a pattern of trenches extends into the body portion from the first surface. The pattern comprises inner trenches that bound each rectifier area and a perimeter trench that has an inside wall extending around the outer perimeter of the plurality of rectifier areas. The trenches accommodate a field-electrode that is connected to the first main electrode. The field-electrode is capacitively coupled to the body portion via dielectric material that lines the trenches so as to provide field-relief regions in the body portion.
The inner trenches are sufficiently closely spaced and the intermediate areas of the body portion are sufficiently lowly doped that the depletion layer formed in the body portion (from the Schottky barrier and from the field-relief regions in the blocking state of the rectifier) depletes the intermediate areas of the body portion between the trenches at a voltage less than the breakdown voltage. In this manner, the trenched inner field-relief regions significantly improve the voltage blocking characteristic of the device.
Premature breakdown of this type of Schottky rectifier can occur at high field points in the depletion layer, especially at the perimeter of the active area. To reduce or avoid such premature breakdown, U.S. Pat. No. 4,646,115 discloses providing this type of rectifier with a perimeter field-relief region comprising a field electrode on dielectric material in a perimeter trench. U.S. Pat. No. 4,646,115 describes forming the perimeter field-relief region simultaneously with the inner field-relief regions so as to reduce the total number of processing steps for the manufacture of the device. In the embodiments shown in U.S. Pat. No. 4,646,115, the perimeter trench is of the same depth and width as the inner trenches. It is lined with the same thickness of the same dielectric material. The perimeter field electrode is present on this dielectric material on inside and outside walls of the perimeter trench (as well the bottom of the trench) and so is capacitively coupled to the body portion across both the inside wall and the outside wall.
It is an aim of the present invention to improve the trenched field-relief regions of Schottky rectifiers, especially at the perimeter of the device, and to facilitate the manufacture of these improved rectifiers.
According to the present invention, there is provided a Schottky rectifier with trenched inner and perimeter field-relief regions. The perimeter field-electrode in its perimeter trench is present on its dielectric material on the inside wall of the perimeter trench so as to be capacitively coupled across said inside wall without acting on any outside wall. Furthermore, the inner and perimeter trenches are sufficiently closely spaced and the intermediate areas of the body portion are sufficiently lowly doped, that the depletion layer formed in the body portion in the blocking state of the rectifier depletes the intermediate areas of the body portion between the trenches at a voltage less than the breakdown voltage. Advantageously the perimeter trench extends deeper in the body than the inner trenches to improve its inwardly directed field relief function.
Thus, in a rectifier in accordance with the invention, the inwardly-acting field electrode of the perimeter trench is so constructed and arranged with respect to the inner trenches as to reduce the high field points by depleting the body portion between the trenches, without any significant outward extension. This depletion arrangement uses the perimeter and inner trenched field-electrodes in a particular form of the so-called xe2x80x9cRESURFxe2x80x9d technique.
Particular advantageous forms of this construction and arrangement can be achieved without requiring extra processing steps in manufacture. In particular, the perimeter trench can be made deeper than the other trenches by making it wider. Due to local loading effects during etching of the inner trenches, this increased width can be used to produce automatically a deeper perimeter trench. A thick dielectric layer is advantageous in the deep perimeter trench and can be provided in various ways.
The invention may be advantageously used in conjunction with various known Schottky rectifier options. Thus, for example, a graded doping can be advantageous in the body portion in some situations, as described in U.S. Pat. No. 5,612,567 and in pending U.S. patent application Ser. No. 09/167,298 which is referenced in columns 11 and 12 of U.S. Pat. No. 5,998,833. The whole contents of U.S. Pat. No. 5,612,567, U.S. Pat. No. 5,998,833 and U.S. application Ser. No. 09/167,298 are hereby incorporated herein as reference material. As described in U.S. Pat. No. 5,998,833, U.S. application Ser. No. 09/167,298 also describes the use of breakdown shielding regions between the perimeter trench and the inner trenches.
Some of the particularly advantageous technical features and some of the options available with the invention are set out in the appended claims. The invention provides several advantageous novel combinations of features, many of which are illustrated in the embodiments now to be described with reference to the drawings. Specific examples are the depth and width of the perimeter trench and its relationship to the perimeter of the semiconductor body, and adjustments in the dopant concentration of the body portion in relation to an increase of dielectric thickness in a lower part of the trench.